1. Field of the Invention
The present invention relates to a fuel cell formed by stacking an electrolyte electrode assembly between a pair of metal separators in a stacking direction. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. A reactant gas supply passage, a reactant gas discharge passage, a coolant supply passage, and a coolant discharge passage extend through the fuel cell in the stacking direction. Further, a reactant gas flow field for supplying a reactant gas along a reaction surface of the electrode is formed between the metal separator and the electrolyte electrode. Further, a coolant flow field for supplying a coolant is formed between the metal separators.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a polymer ion exchange membrane as a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. Each of the anode and the cathode is made of electrode catalyst and porous carbon. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form the fuel cell. In use, generally, a predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. An oxidizing gas (reactant gas) such as a gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode membrane, creating a DC electrical energy.
In the fuel cell, for example, a metal plate is used for fabricating the separator. The strength of the metal separator is high in comparison with a carbon separator, and the metal plate is suitable for fabricating a thin separator. Reactant gas flow fields having the desired shapes are formed on the metal separator by press forming in order to reduce the thickness of the metal separator, and to achieve reduction in the overall size and weight of the fuel cell.
Seal members are formed integrally on the metal separator, and the metal is partially exposed on the cooling surface for controlling the temperature of the electrode reaction surface of the membrane electrode assembly facing the metal separator. For example, Japanese Laid-Open Patent Publication No. 11-129396 discloses a method of producing a fuel cell separator 3 as shown in FIG. 7. According to the disclosure, a seal member 2 is formed integrally on a metal separator body 1 by injection molding.
The fuel cell separator 3 has a cooling surface and a reaction surface opposite to the cooling surface. On the cooling surface, metal of the separator body 1 is exposed in a central region 4 of the fuel cell separator 3. A coolant flow field 6 is formed by corrugated patterns 5 in the central region 4. The coolant flow field 6 is connected to fluid passages 8a, 8b through channels 7a, 7b at diagonal positions. The reaction surface faces an electrode reaction surface 9 of an electrode (not shown).
In the conventional technique, the surface area of the exposed metal in the central region 4 of the separator body 1 is larger than the surface area of the electrode reaction surface 9. Therefore, temperature distribution in the electrode reaction surface 9 is large. Consequently, variation occurs in the power generation performance of the unit cells (fuel cells) of the fuel cell stack.